Setting Primary Key Fields

Tables often contain one or more fields whose values differentiate individual records in the table from the other records. These fields are called key fields or, more precisely, primary key fields.

Primary key fields always contain unique values in each record; that is, the same value can't occur twice in a primary key field in a table. Keys are an integral part of relational databases—by ensuring that each record refers to a unique entity, keys allow you to create relationships between tables. Car license plates are a perfect example of a primary key field value; no two cars (in the same state, at least) will have the same license plate numbers. For more information on relationships, see Chapter 8.